1. Field of the Invention
The present invention relates to a MEMS (Micro-Electro-Mechanical System) resonator formed using MEMS technique capable of realizing an ultra-fine mechanical mechanism using a fine processing technique for semiconductors, and also, the present invention relates to a MEMS oscillation circuit using the MEMS resonator, and a MEMS device using the MEMS resonator.
2. Description of the Related Art
Recently, demand for RF technology has increasingly risen. Various requirements are made of RF devices to follow functional diversification and a sharp increase in the number of users of the RF devices. It is particularly desired to provide a multi-clock function for directly generating a plurality of frequencies in addition to downsizing and low cost to an oscillator because of the need to make effective use of limited RF frequencies. In such social background, attention has been paid to the MEMS technique for application of portable wireless terminal devices. This is because a MEMS device is characterized by low power consumption, high density packaging, broadband characteristics, and the like.
A MEMS resonator has been actively studied mainly in the U.S.A. since the latter half of 1990s. At present, a few companies have now been able to provide MEMS resonator samples. These products mainly replace crystal oscillators and can characteristically downsize devices. On the other hand, possibility of the MEMS resonator is not limited to simple replacement of the crystal oscillator. For example, the multi-clock function of the MEMS resonator is expected to create a new market differently from overtone technique related to the crystal oscillators.
Documents related to the present invention are as follows:    Patent Document 1: Specification of U.S. Pat. No. 6,490,147;    Patent Document 2: Japanese patent laid-open publication No. JP-2007-184931-A1;    Non-Patent Document 1: Wan-Thai Hsu, “Vibrating RF MEMS for Timing and Frequency References”, Digests of IEEE MTT-S 2006 International Microwave Symposium, pp. 672-675, 2006; and    Non-Patent Document 2: Kun Wang et al., “VHF Free-Free Beam High-Q Micromechanical Resonator”, Journal of Microelectromechanical Systems, Vol. 9, No. 3, pp. 347-360, September 2000.
For example, Non-Patent Document 1 describes outline of a vibrating RF-MEMS oscillator. In this case, types of the RF-MEMS oscillator are classified into a beam type, a disk type, a ring type, and an FBAR (Film Bulk Acoustic Resonators) type. Among these types, a “free-free beam MEMS resonator” disclosed in Non-Patent Document 2 (which resonator is also described in, for example, Patent Documents 1 and 2) will be now described with reference to FIG. 18.
As shown in FIG. 18, the “free-free beam MEMS resonator” according to a prior art is configured as follows. A central beam 100 (called “free-free resonating beam”) supported by a ground surface and four springs 101 fixed to anchors 102, respectively, on a detection electrode 90 is provided in a lower portion of the center of the resonator. The central beam 100 is bent and oscillates by receiving an electrostatic force from an electrode 91 excited by an alternating-current (AC) signal Vi. In FIG. 18, reference symbol 103 denotes a concave portion, reference symbol 104 denotes a node point in a flexural mode, and an oscillation signal output circuit 110 is connected to the detection electrode 90. A basic oscillation mode of the resonator is a mode in which the central beam 100 flexes vertically. Parts having high amplitudes (oscillation centers) are located at three portions, i.e., the center and both ends of the resonator, respectively. In view of waveform, this oscillation mode corresponds to a wave having a half-wavelength. Further, the Non-Patent Document 1 reports that devices of various dimensions are designed and that a device having a resonant frequency of 30 MHz to 90 MHz is produced as a prototype.
However, the “free-free beam MEMS resonator” according to the prior art has the following problems. A resonant frequency of the resonator is decided by dimensions of a device. For example, when a length of the resonating beam is set to 16 μm and 14 μm, corresponding resonance frequencies are 50 MHz and 70 MHz, respectively. In this way, according to the prior art, only one resonant frequency can be pulled out from one device. The reason is as follows. Generally speaking, a resonator can excite an oscillation of a higher harmonic wave that is an integer multiple of a fundamental wave. However, since the support springs supporting the oscillating beam function to suppress harmonics, the frequency pulled out from the resonator is limited to a constant frequency. As can be seen, a structure of the resonator according to this prior art is disadvantageously incapable of changing frequencies.
Moreover, the electrostatic force generated from the electrode provided to be opposed to the oscillating beam directly acts on the oscillating beam. Therefore, as the frequency is higher, the oscillating beam is smaller in dimensions and an area of the oscillating beam opposed to the exciting electrode is made smaller. As a result, it is disadvantageously difficult for the electrostatic force to effectively act on the oscillating beam and driving voltage disadvantageously increases.
On the other hand, in a manner different from the structure according to the prior art in which the four support springs 101 are provided in portions that are nodes of traverse vibrations corresponding to modified directions of the oscillating beam, respectively, there has been known another structure of fixing both ends of an oscillating beam according to another prior art. In this case, it is possible to use a harmonic wave in addition to the fundamental wave. However, the structure according to another prior art has a problem that the harmonic wave is always low in amplitude than a lower-order harmonic wave. Accordingly, although a plurality of resonance frequencies can be used by use of the overtone technique, strains occur to an oscillation waveform as a higher-order resonant frequency is used. Furthermore, the structure according to another prior art also has a problem of need of a filter circuit suppressing lower-order frequency.